<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<!-- Copyright (C) 1992-2017 Free Software Foundation, Inc.
Contributed by Cygnus Support.  Written by Julia Menapace, Jim Kingdon,
and David MacKenzie.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
Texts.  A copy of the license is included in the section entitled "GNU
Free Documentation License". -->
<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
<head>
<title>STABS: ELF Linker Relocation</title>

<meta name="description" content="STABS: ELF Linker Relocation">
<meta name="keywords" content="STABS: ELF Linker Relocation">
<meta name="resource-type" content="document">
<meta name="distribution" content="global">
<meta name="Generator" content="makeinfo">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<link href="index.html#Top" rel="start" title="Top">
<link href="Symbol-Types-Index.html#Symbol-Types-Index" rel="index" title="Symbol Types Index">
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
<link href="Stab-Sections.html#Stab-Sections" rel="up" title="Stab Sections">
<link href="GNU-Free-Documentation-License.html#GNU-Free-Documentation-License" rel="next" title="GNU Free Documentation License">
<link href="Stab-Section-Basics.html#Stab-Section-Basics" rel="prev" title="Stab Section Basics">
<style type="text/css">
<!--
a.summary-letter {text-decoration: none}
blockquote.smallquotation {font-size: smaller}
div.display {margin-left: 3.2em}
div.example {margin-left: 3.2em}
div.indentedblock {margin-left: 3.2em}
div.lisp {margin-left: 3.2em}
div.smalldisplay {margin-left: 3.2em}
div.smallexample {margin-left: 3.2em}
div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
div.smalllisp {margin-left: 3.2em}
kbd {font-style:oblique}
pre.display {font-family: inherit}
pre.format {font-family: inherit}
pre.menu-comment {font-family: serif}
pre.menu-preformatted {font-family: serif}
pre.smalldisplay {font-family: inherit; font-size: smaller}
pre.smallexample {font-size: smaller}
pre.smallformat {font-family: inherit; font-size: smaller}
pre.smalllisp {font-size: smaller}
span.nocodebreak {white-space:nowrap}
span.nolinebreak {white-space:nowrap}
span.roman {font-family:serif; font-weight:normal}
span.sansserif {font-family:sans-serif; font-weight:normal}
ul.no-bullet {list-style: none}
-->
</style>


</head>

<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
<a name="ELF-Linker-Relocation"></a>
<div class="header">
<p>
Previous: <a href="Stab-Section-Basics.html#Stab-Section-Basics" accesskey="p" rel="prev">Stab Section Basics</a>, Up: <a href="Stab-Sections.html#Stab-Sections" accesskey="u" rel="up">Stab Sections</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Types-Index.html#Symbol-Types-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Having-the-Linker-Relocate-Stabs-in-ELF"></a>
<h3 class="appendixsec">F.2 Having the Linker Relocate Stabs in ELF</h3>

<p>This section describes some Sun hacks for Stabs in ELF; it does not
apply to COFF or SOM.  While <small>GDB</small> no longer supports this hack
for Sun Stabs in ELF, this section is kept to document the issue.
</p>
<p>To keep linking fast, you don&rsquo;t want the linker to have to relocate very
many stabs.  Making sure this is done for <code>N_SLINE</code>,
<code>N_RBRAC</code>, and <code>N_LBRAC</code> stabs is the most important thing
(see the descriptions of those stabs for more information).  But Sun&rsquo;s
stabs in ELF has taken this further, to make all addresses in the
<code>n_value</code> field (functions and static variables) relative to the
source file.  For the <code>N_SO</code> symbol itself, Sun simply omits the
address.  To find the address of each section corresponding to a given
source file, the compiler puts out symbols giving the address of each
section for a given source file.  Since these are ELF (not stab)
symbols, the linker relocates them correctly without having to touch the
stabs section.  They are named <code>Bbss.bss</code> for the bss section,
<code>Ddata.data</code> for the data section, and <code>Drodata.rodata</code> for
the rodata section.  For the text section, there is no such symbol (but
there should be, see below).  For an example of how these symbols work,
See <a href="Stab-Section-Transformations.html#Stab-Section-Transformations">Stab Section Transformations</a>.  GCC does not provide these symbols;
it instead relies on the stabs getting relocated.  Thus addresses which
would normally be relative to <code>Bbss.bss</code>, etc., are already
relocated.  The Sun linker provided with Solaris 2.2 and earlier
relocates stabs using normal ELF relocation information, as it would do
for any section.  Sun has been threatening to kludge their linker to not
do this (to speed up linking), even though the correct way to avoid
having the linker do these relocations is to have the compiler no longer
output relocatable values.  Last I heard they had been talked out of the
linker kludge.  See Sun point patch 101052-01 and Sun bug 1142109.  With
the Sun compiler this affects &lsquo;<samp>S</samp>&rsquo; symbol descriptor stabs
(see <a href="Statics.html#Statics">Statics</a>) and functions (see <a href="Procedures.html#Procedures">Procedures</a>).  In the latter
case, to adopt the clean solution (making the value of the stab relative
to the start of the compilation unit), it would be necessary to invent a
<code>Ttext.text</code> symbol, analogous to the <code>Bbss.bss</code>, etc.,
symbols.  I recommend this rather than using a zero value and getting
the address from the ELF symbols.
</p>
<p>Finding the correct <code>Bbss.bss</code>, etc., symbol is difficult, because
the linker simply concatenates the <code>.stab</code> sections from each
<samp>.o</samp> file without including any information about which part of a
<code>.stab</code> section comes from which <samp>.o</samp> file.  The way GDB use to
do this is to look for an ELF <code>STT_FILE</code> symbol which has the same
name as the last component of the file name from the <code>N_SO</code> symbol
in the stabs (for example, if the file name is <samp>../../gdb/main.c</samp>,
it looks for an ELF <code>STT_FILE</code> symbol named <code>main.c</code>).  This
loses if different files have the same name (they could be in different
directories, a library could have been copied from one system to
another, etc.).  It would be much cleaner to have the <code>Bbss.bss</code>
symbols in the stabs themselves.  Having the linker relocate them there
is no more work than having the linker relocate ELF symbols, and it
solves the problem of having to associate the ELF and stab symbols.
However, no one has yet designed or implemented such a scheme.
</p>
<hr>
<div class="header">
<p>
Previous: <a href="Stab-Section-Basics.html#Stab-Section-Basics" accesskey="p" rel="prev">Stab Section Basics</a>, Up: <a href="Stab-Sections.html#Stab-Sections" accesskey="u" rel="up">Stab Sections</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Types-Index.html#Symbol-Types-Index" title="Index" rel="index">Index</a>]</p>
</div>



</body>
</html>
